The liquid crystal display (or LCD) is made up of two substrates and a liquid crystal layer interposed therebetween. The light is transmitted under the control of the electric field intensity applied to the liquid crystal layer.
The twisted nematic (TN) liquid crystal display, which is currently the most popular LCD, has a transparent first substrate and a transparent second substrate, a pair of transparent electrodes respectively formed on the inner surface of the transparent substrates and opposite to each other so as to drive the liquid crystal layer interposed therebetween, and a pair of polarizing plates which are respectively attached to the outer surfaces of the transparent substrates. In the off state of the LCD, that is, in the state that the electric field is not applied to the transparent electrodes, the orientations of the liquid crystal molecules are aligned perpendicular to the substrates.
Unfortunately, the contrast ratio of the conventional TN LCD in a normally black mode may not be so high because the incident light is not fully blocked in the off state. In order to obviate this problem and to increase the viewing angles of LCD, various LCD modes have been presented. An example of the new LCD mode is known as vertical alignment (VA) mode. As the name suggests, the liquid crystal molecules are normally aligned perpendicular to the inner surface of the substrates, swinging through 90° to lie parallel with substrates in the presence of electric field. This LCD mode produces a display with an ultra-wide viewing angle and high contrast ratio but with the added bonus of higher brightness and a response time of 25 milliseconds. In addition, this LCD mode also consumes less power.
Following the advent of VA mode LCD, a new technique was proposed to align the liquid crystal molecules at a sub-level which uses UV light instead of the usual rubbing. This technique involves the addition of pyramid-shaped protrusions with each of liquid crystal cell, the surface of which each makes up a separate domain, in which the liquid crystal molecules are aligned differently from those in other domains. It produces increased viewing angles, at the expense of a reduction in brightness, by ensuring that each of the multiple domains within a pixel cell channel light at an angle to the substrates, instead of at right angles to it. The result is an all-round increase in viewing angle with no variation in color tone as the viewing angle increases and, requiring no rubbing, a simplified manufacturing process with a reduction in the possibility of liquid crystal contamination. When combined with the VA mode, the resultant display is known as a multi-domain vertical alignment (MVA) mode LCD and produces a viewing angle of 160° in all directions with a high contrast ratio of around 300:1.
However, the pyramid-shaped protrusions which are applied to control the tilt direction of the liquid crystal molecules are the major reasons for the low yield and high cost of the LCD products. There is an inclination to develop an active matrix LCD which has an improved response time, an increased viewing angle, an enhanced yield and a lower cost.